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International Journal of Ambient Energy, 7, 4, pp. 191-196, 1986-10
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Natural convection in the cavity of a basement block wall
Svec, O. J.; Goodrich, L. E.
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https://nrc-publications.canada.ca/eng/view/object/?id=b1e1eaad-5bdc-4fd8-8884-821dbcf0531f https://publications-cnrc.canada.ca/fra/voir/objet/?id=b1e1eaad-5bdc-4fd8-8884-821dbcf0531fNatural Convection in the Cavity of a
Basement Block Wall
by O.J. Svec and L.E. Goodrich
Reprinted from
International Journal of Ambient Energy
Vol. 7, No. 4, October 1986
p. 191 - 196
(IRC Paper No. 1475)
Price
$3.00
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N R C-
CISTII R C
L I B R A R Y
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Un programme d ' e x p g r i e n c e e n v r a i e g r a n d e u r p o r t a n t s u r l e s probl8mes de s o u l ~ v e m e n t dO a u g e l d ' u n s o u s - s o l i s o l ' e e s t a c t u e l l e m e n t e n c o u r s a u C o n s e i l n a t i o n a l d e r e c h e r c h e s du Canada. En p l u s d e s donn'ees r e c u e i l l i e s s u r l e soul8vement, c e t t e e x p e r i e n c e a permis d e t i r e r c e r t a i n e s c o n c l u s i o n s i n t ' e r e s s a n t e s c o n c e r n a n t l e convexion n a t u r e l l e
3
l ' i n t ' e r i e u rd e l a c a v i t g d e s murs e n b l o c s d e beton. Les donn'ees e x p g r i m e n t a l e s e t un c a l c u l s i m p l i f i g u t i l i s a n t l a mgthode d e s 616ments f i n i s o n t t o u s deux d'emontr'e que l a convexion n a t u r e l l e d a n s un mur d e b l o c s d e s o u s - s o l p e u t c o n s t i t u e r u n f a c t e u r i m p o r t a n t d a n s l e s d g p e r d i t i o n s c a l o r i f i q u e s . La p r a t i q u e a c t u e l l e c o n s i s t a n t 3 n ' i s o l e r q u e l a p a r t i e s u p ' e r i e u r e i n t g r i e u r e d e s murs n ' e s t p e u t - t t r e p a s e f f i c a c e p o u r l i m i t e r l e s p e r t e s d e c h a l e u r p a r l e s murs d e b l o c s , m a i s l ' i s o l a t i o n du mur s u r t o u t e l a h a u t e u r p e u t c a u s e r une r g d u c t i o n c o n s i d e r a b l e d e s t e m p e r a t u r e s a u n i v e a u d e s s e m e l l e s .
International Journal of Ambient Energy, Volume 7, Number 4 October 1986
Natural
convection in
the cavity of a
basement
block wall
0.
J.
Svec* and
L.
E. Goodrich
*
* 0. J. Svec and L. E. Goodrich, Institute for Research in Construction, National Research Council o f Canada, Ottawa. Canada.
International Journal of Ambient Energy, Volume 7, Number 4 October 1986
Natural
convection in
the cavity of a
basement
block wall
0. J. Svec* and L. E. Goodrich
*
* 0. J. Svec and L. E. Goodrich, Institute for Research in Construction, National Research Council of Canada, Ottawa, Canada.
O Crown Copyright 1986.
SYNOPSIS
A full-scale experimental programme to study the a d f r e e z e l h e a v i n g p r o b l e m o f a n i n s u l a t e d basement is currently underway at t h e National Research Council of Canada. Apart f r o m heaving r e s u l t s t h i s e x p e r i m e n t has a l s o y i e l d e d i n t e r e s t i n g f i n d i n g s r e l a t e d t o t h e natural convection in concrete-block wall cavities. Both the field data and a simplified finite element model calculation s h o w that natural convection in a block basement wall can be a significant factor contributing t o heat losses. Insulating only the inside upper portion of the wall, as is current practice, may n o t be effective in reducing heat losses w i t h block walls, w h i l e f u l l l e n g t h insulation may lead to significant lowering of ground temperatures near footing levels.
INTRODUCTION
D u r ~ n g the last decade energy conservation became an important factor for the construction industry. The problem of heat losses through all components of a building envelope has received considerable a t t e n t i o n . A large a m o u n t o f research work has been done on heat losses f r o m basements, taking into account various types of wall construction, insulation, type of soil surrounding basements, soil moisture content together w i t h soil thermal characteristics and, of course, the e f f e c t of climatological conditions
[I-71. The e f f e c t of natural convection in t h e cavity of a block wall has not, however, been sufficiently addressed. This paper examines the overall heat f l o w through a block wall as w e l l a t h e consequences of enhanced wall internal cooling by natural convection.
PROBLEM STATEMENT
I n accordance w i t h t h e Canadian National Building Code, basements in all n e w buildings m u s t b e i n s u l a t e d . T h e v a s t m a j o r i t y o f b a s e m e n t s are i n s u l a t e d f r o m t h e i n s i d e , because of the greater cost of placing styrofoam insulation on t h e outside surface of basement walls, especially in retrofit situations. I n order t o save energy there has been a tendency t o lower the air temperature in buildings in general and in basements in particular. While such conservation measures considerably reduce t h e heat f l o w through basement walls, they also cause t h e zero degree isotherm (frost line) t o move further into the wall itself. The interface b e t w e e n t h e wall and surrounding soil w i l l thus be at a subfreezing temperature, possibly allowing soil t o freeze solidly t o the wall. If t h e soil is frost susceptible (clay or silt) and sufficient moisture is available, frost heaving could potentially l i f t part of the wall. Because typical cast-in-place concrete basement walls have small or negligible tensile strength t o resist t h e shear stress on the wall-soil interface imposed by heaving, the risk
Natural convection in the cavity of a basement block wall Svec, Goodrich
of damage exists. Concrete-block walls are, of course, even weaker.
In the case of a block wall insulated or partly insulated on its inside surface, there can be a large temperature difference b e t w e e n the inside and outside boundaries of the wall cavity. This temperature difference creates a strong air f l o w in the wall cavity, which can substantially cool the lower part of the wall by natural convection. The cooling may b e sufficient t o create conditions favourable for frost heave in t h e soil near the base of t h e wall. If, however, the insulation covers only t h e upper inside wall surface, as is f r e q u e n t l y done, h e a t l o s s resulting f r o m the enhanced convection may be substantial.
EXPERIMENT
To investigate these potential problems in a field situation an experimental house was constructed on the National Research Council of Canada ( N R C C ) c a m p u s a t O t t a w a . A c o m p l e t e description of t h e equipment along w i t h thermal and movement data collected over three seasons w i l l be presented elsewhere [8] and only a short description of the relevant parts of that project w i l l be given here.
All four walls of the experimental basement ( d i m e n s i o n s 10 m x 10 m ) w e r e c o n s t r u c t e d w i t h s l i p j o i n t s i n t h e c o r n e r s t o a l l o w independent wall movements (Figure 1). Two of the walls were built of concrete blocks and t w o of cast-in-place concrete. Both block walls w e r e insulated w i t h 10 c m of styrofoam on the inside surface. One concrete wall and one block wall were insulated full height while t h e other block wall was insulated only from the t o p t o a level 60 cm below the outside grade. The second
BLOCK WALL D l A L GAUGES HORIZONTAL & VERTICAL INSTRUh~ENTATION THERMISTORS FRAA'E CONCRETE
Figure 1 Plan view of the experimental basement.
concrete wall (facing south) was n o t used in t h e e x p e r i m e n t . T h e s e t h r e e w a l l s w e r e instrumented w i t h thermistors for temperature monitoring and w i t h dial gauges for horizontal and vertical displacement measurements (see Figure 1 ) . Thermistors w e r e backed u p b y t h e r m o c o u p l e s a t s e l e c t e d c r i t i c a l p o i n t s . G r o u n d t e m p e r a t u r e s a n d v e r t i c a l s o i l movements next t o the wall w e r e measured, and apparatus f o r periodic in-situ monitoring o f thermal properties and moisture contents w e r e a l s o i n s t a l l e d . T h e r m i s t o r o u t p u t s w e r e automatically recorded at 4 h intervals using a DEC 1 1-34 minicomputer, while .dial gauges w e r e periodically read manually for possible v e r t i c a l ( h e a v e ) a n d h o r i z o n t a l ( c a v e - i n ) movements.
E x p e r i m e n t a l Results. Typical early morning mid-winter temperature distributions in the t w o block walls and t h e concrete wall are shown in Figure 2. A significant difference can be seen b e t w e e n partly and fully insulated block walls, where the 0°C isotherm i n the partly insulated wall is considerably nearer the ground surface, presumably because of the large heat losses caused by natural convection. The temperature difference b e t w e e n the inside of t h e lower part of t h e uninsulated wall (+13.5"C at A, Figure 2) and t h e upper exposed part of t h e wall (-13°C at B) creates a strong convective cell w i t h i n t h e w a l l cavity. The 0°C i s o t h e r m i n t h e fully insulated block wall is at approximately the same location as in t h e fully insulated concrete wall. This location is largely conditioned by t h e t e m p e r a t u r e o f t h e s u r r o u n d i n g s o i l . T h e temperatures at the base of t h e walls are, however, distinctly different. The cause of this is again, presumably, natural convection in the block wall cavity.
For t h e c o n c r e t e w a l l , e x t e r i o r b a s e temperatures are similar t o or just slightly warmer than ambient soil temperatures at the same level. Base temperatures for the partly insulated block w a l l are approximately 3°C warmer than ambient conditions and are the w a r m e s t o f t h e t h r e e c a s e s . E x t e r i o r temperatures at the base of the fully insulated block wall are, by contrast, about 3°C colder than the ambient soil temperatures.
Typical thermal behaviour of the walls w i t h t i m e is i n d i c a t e d i n F i g u r e s 3 a n d 4. T e m p e r a t u r e s a t a s i m i l a r e l e v a t i o n (approximately 30 cm below ground level) on both inside and outside faces of the concrete are s h o w n v e r s u s t i m e f o r t h e t h r e e w a l l configurations. Figure 3 presents the cooling trend, and Figure 4 the warming trend. The temperature difference between the t w o wall surfaces for t h e block wall remains roughly constant ( 2
-
2.5"C) throughout t h e winter. ThisNatural convection in the cavity of a basement block wall Svec, Goodrich C A S T - I N - P L A C E W A L L B L O C K W A L L B L O C K W A L L Figure 2 Temperature distribution and 0°C isotherm in basement walls and surrounding ground on January 12, 1984.
is in sharp contrast w i t h t h e situation in t h e aT
concrete wall w h e r e horizontal temperature Conservation of Energy: PC-
at
- . = -*
a x ;+
Qdifferences, far f r o m remaining near c-onstant, even showed reversal on t w o occasions. In spite
Constitutive Relations: o f t h e v a r v i n a a m b i e n t c o n d i t i o n s , t h e
temperature differences at similar levels in t h e
~ 1= j Paii
+
p -t w o block walls remained nearly constant. This
(:::
+3)
behaviour is further evidence of the important
convective transfer occurring in these cases. q . = - k - aT ax,
FINITE ELEMENT M O D E L
A general purpose, finite element program for s t u d y i n g n a t u r a l c o n v e c t i o n p h e n o m e n a developed by Gartling [9] has been used in this w o r k . T h e f o l l o w i n g a s s u m p t i o n s a r e implemented in this model: only buoyancy- induced motion and heat transfer are considered in the block wall cavity, while conductive heat transfer alone is assumed in all solid materials. Air in t h e block cavity is assumed t o be a Newtonian and incompressible fluid, t h e f l o w t o be two-dimensional and laminar, and viscous dissipation t o b e negligible. The f o l l o w i n g system of equations was used:
Conservation of Mass:
*=
o
axiau
aTii
Conservation of M o m e n t u m : p 2 a t = - ax,+
p gwhere u is the velocity vector, T the time, P the
pressure, T the temperature, p t h e density, T , ~
the stress tensor, q the heat flux vector, Q the volumetric heat source, p the viscosity, c t h e specific heat, k the thermal conductivity, and
P
the coefficient of volume expansion.
The computer program was developed using the Galerkin method t o f o r m the corresponding discretization. Formulation of t h e program is based on isoparametric elements. This program makes u s e o f e i t h e r t h e N e w t o n - R a p h s o n procedure or t h e Picard algorithm as iteration procedure during equation solution. The frontal method is employed for solution of the matrix equations. Depending on t h e type of problem under consideration the program can handle i s o t h e r m a l , w e a k l y c o u p l e d c o n v e c t i o n or
I
Natural convection in the cavity of a basement block wall Svec, Goodrich -6. 0 6 7 8 9 1 0 11 1 2 1 3 J A N U A R Y . 1 9 8 4-
I
I
I
1
I
I
CAST-IN- 1 II
I
1
-
-
-&----
-
-
-
-
-
'
.
-
-
-
-
-
-
-
-
-
-
+-
-
-
-
Figure 3 Temperatures of wall surfaces during cooling trend.strongly coupled convection. The details of the block wall cavity. The objective was not t o p r o g r a m a n d f i n i t e e l e m e n t m e t h o d o l o g y validate t h e c o m p u t e r model b u t rather t o
employed are described fully in [9]. compare predicted temperature distributions in a
qualitative sense w i t h those measured in the
Finite Element Results. The purpose of the field and, t o gain an insight into air movement in finite element analysis was t o s h o w the main t h e wall cavity.
trend in t h e temperature distribution as w e l l as The finite element mesh consisted o f 1 1 1
t h e behaviour of t h e air f l o w (streamlines) in the 8-noded quadratic isoparametric elements. The
- 6 . 0
1 0 11 1 2 1 3 1 4 1 5
F E B R U A R Y , 1 9 8 4
Figure 4
Temperatures of w d l surfaces during warming trend.
Natural convection in the cavity of a basement block wall Svec, Goodrich
Figure 5
Results of finite element modelling in steady state;
left temperature isotherms
in a basement block wall;
right streamlines in the
block wall cavity.
i cavity wall was represented by a single block, consisting of outer and inner vertical concrete members containing an air-filled cavity. The concrete members w e r e modelled as single solid elements whereas the block wall cavity was modelled by a 4 x 1 1 element mesh.
Boundary conditions used in t h e steady-state analysis together w i t h computed temperature isotherms are shown in Figure 5a, while air-flow s t r e a m l i n e s a r e p r e s e n t e d i n F i g u r e 5 b . Calculated temperatures are similar t o those measured in t h e experiment. Temperatures on the outside of the wall at the foundation level
c o m p a r e r a t h e r w e l l ; 6 . I 0 C a n d 5.7"C. respectively. Form as w e l l as location of m o s t of the isotherms are also similar. An exception is the 0°C isotherm, which, in the field, is located higher than predicted. This is due t o the strong i n f l u e n c e o f l a t e n t h e a t i n t h e s o i l ( n o t considered in t h e model) as w e l l as t o t h e transient nature of the actual field situation.
Figure 5b s h o w s the convective cell in t h e block wall cavity. I t is interesting t o observe t h a t w i t h i n t h e main convective cell spanning t h e entire cavity t w o smaller secondary convective cells developed. This is probably caused by t h e
-
Natural convection in the cavity of a basement block wall Svec, Goodrich
distinct difference in outside wall temperature 2. Mitalas, G. P. "Basement heat loss studies between the upper exposed surface and the at DBRINRC" DBR Paper No. 1045, Division
lower buried portion. of Building Research, National Research
Council of Canada, Ottowa, Ontario, Canada,
CONCLUSION
As a result of this study, certain important, as well as practical, conclusions can be drawn. If basement walls are constructed using concrete b l o c k s , t h e n any o p p o r t u n i t y f o r n a t u r a l convection to develop in the wall cavity will significantly increase the overall heat losses. With fully insulated block walls, considerable cooling can occur near the base of the wall, which raises the possibility of causing frost heave damage to the footings in appropriate c i r c u m s t a n c e s . T h e r e f o r e , i f i n s u l a t i o n is extended right down to the basement floor, i t is imperative that the natural convection in the wall be prevented or, at least, significantly retarded. Insulation covering only the upper inside surface may result in high heat losses owing t o the enhanced convection made possible by this configuration.
I t can be inferred as well that exterior p l a c e m e n t o f t h e insulation w o u l d reduce temperature gradients and hence convection within the wall cavity. Heat losses would be correspondingly diminished while, at the same t i m e , f o o t i n g t e m p e r a t u r e s w o u l d n o t b e lowered by the presence of the insulation.
These and other questions arising out of this work w i l l continue t o be examined in the future.
REFERENCES
1. Houghten, F. C., Taimuty, S. K., Gutberlet, C. and B r o w n , C. J . " H e a t l o s s t h r o u g h basement walls and floors" Heating, Piping and Air-Conditioning Vol. 14 1942 pp. 69-74.
1982.
Backstrom, H. "Loss of energy in t h e foundations of detached houses" Swedish Council for Building Research, Stockholm, Report R32.
S w i n t o n , M . C. and P l a t t s , R. E. "Engineering method for estimating annual b a s e m e n t h e a t l o s s a n d i n s u l a t i o n p e r f o r m a n c e " T r a n s a c t i o n s , A m e r i c a n S o c i e t y o f Heating, Refrigerating, and Air-Conditioning Engineers, 87, Pt. 2, 1981, pp. 343-359.
Manian, V. S. "Heat loss through basement walls" Hydro-Electric Power Commission of Ontario, Research Division, Report 69-343-K, December 16,1969.
McBride, M . F., Blancett, R. S., Sepsy, C. F. and Jones, C. D. "Measurement of subgrade temperatures for prediction of heat loss i n b a s e m e n t s " Transactions, American Society of Heating, Refrigerating, and Air- Conditioning Engineers, 85, Part I, 1979, pp. 642-654.
Claesson, J . and Eftring, B. " O p t i m a l distribution of thermal insulation and ground heat loss" Swedish Council for Building Research, Stockholm, Document D33: 1980. Goodrich, L. E. and Svec, 0. J. "Soil freezing and t h e r m a l r e g i m e m e a s u r e m e n t s i n insulated basement walls" NRCC Research Report (in preparation).
Gartling, D. K. "NACHOS
-
A finite element computer program for incompressible f l o w p r o b l e m s " Sandia National Laboratory, SAND77-1333,1978.T h i s p a p e r i s being d i s t r i b u t e d i n r e p r i n t form by t h e I n s t i t u t e f o r R e s e a r c h i n C o n s t r u c t i o n . A l i s t of b u i l d i n g p r a c t i c e and r e s e a r c h p u b l i c a t i o n s a v a i l a b l e from t h e I n s t i t u t e may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n , I n s t i t u t e f o r R e s e a r c h i n C o n s t r u c t i o n , N a t i o n a l Research C o u n c i l of C a n a d a , O t t a w a , O n t a r i o ,
K1A 0R6.
C e document e s t d i s t r i b u 6 sous forme de t i r 6 - 3 - p a r t p a r 1' I n s t i t u t de r e c h e r c h e e n c o n s t r u c t i o n . O n peut o b t e n i r une l i s t e d e s p u b l i c a t i o n s de 1 ' I n s t i t u t p o r t a n t s u r l e s t e c h n i q u e s ou les r e c h e r c h e s e n
